Chris McMahon calls them "involuntary collaborations". He buys fairly dull landscapes at yardsales and then adds monsters to them. To give credit to the first artist, he just adds his own name in the bottom right corners. You can view two others at the link.

Reg Mombassa ( T-Shirt Mambo Artist ) did this in the 80's to cheap motel paints while he was touring with the band Mental As Anything. he would hang the back on the wall , but now they are collectors items.